Electro-optically active organic diodes are for example used as organic light emitting diodes (OLEDs), in lightning devices, in display devices and in organic solar cell devices. An organic diode in an organic solar cell device is arranged to generate electricity from light, whilst in a lightning device, the organic diode is arranged to generate light from electricity. Nevertheless, these are just different manifestations of common fundamental properties pertaining to certain electro-optically active organic materials. Progress and development in one area, such as in the area of lightning devices and OLEDs, thus can be utilized for improvements in the other area, such as in the area of organic solar cell devices.
This far, efforts have mostly been spent on devices for generation of light, and in particular on OLEDs. This is partly owing to that the obtainable efficiency, reliability and operational lifetime so far have been considered too poor for viable organic solar cell devices, especially in light of what can be achieved in conventional solar cell devices. Although these properties are desirable to improve also in the area of lightning application devices, the requirements are often not fully as high, and there are already commercially available products, such as displays based on OLED technology. Partly, this is owing to that OLEDs emit light and thus do not require backlighting such as in conventional liquid crystal displays (LCDs). Some other advantages of electro-optically active organic diodes in general are for example that they are comparatively easy and cost efficient to make, can be fabricated in thin, flexible layers and even be made transparent.
Recent electro-optically active organic diodes have shown great progress, especially in efficiency and lifetime, however the increased lifetime has in some cases lead to problems with reliability. Reliability is a problem in general in the area of electro-optically active organic diodes and there is a general need for improved operational life times. Although many of the desired and beneficial properties are owing to the organic nature, there are also some drawbacks that follow from this, for example, organic materials are in general more sensitive to harsh physical treatments and high temperatures compared to many inorganic materials.
So called stacked OLED structures have been presented, which for example are used to improve performance. These are also known as tandem, or cascaded, OLEDs. A stacked OLED comprises several organic layers, each comprising at least one emissive, active layer. The organic layers in a stacked OLED are also known as organic units, organic electroluminescent units, organic EL units or simply EL units. The EL units in a stacked OLED are arranged between a common anode and a common cathode. Additionally, in order to be efficient, there is typically a so called connection layer arranged between the EL units. Connection layers are also known connection units, intermediate connectors or connector units.
Known connection layers include organic layers, which typically attain good transparency. Since the light emitted in an OLED has to be let out, good transparency is in general an important property. However, due to the increased number of layers in a stacked OLED compared to an ordinary, single EL unit OLED, transparency becomes even more important. Too low transparency in the additional layers may result in that the stacked OLED provides even less light than a corresponding single EL unit OLED.
The prior art also includes use of inorganic connection layers.
US 20050264174 discloses a tandem OLED comprising an inorganic connector unit having a layer structure of three layers; a low work function (<4 eV) metal layer towards the anode side, a metal compound p-type semiconductor layer towards the cathode side and a high work function (>4 eV) metal layer in-between.
However, owing to the nature of metals and the number of the layers in this structure, the transparent properties are poor. Metals in general have comparatively high light absorption and reflectivity. The latter may for example also result in undesired optical cavity effects. To some extent, problems related to metals can be alleviated by very thin layers, however, absorption and reflectivity may still be comparatively high, and transparency will still be a problem when the number of EL units increase. Also, doping, such as in a p-type semiconductor, often have detrimental influence on transparency.
In conclusion, there is a general desire for efficient electro-optically active organic diodes and increased reliability, and there is also, in particular for stacked electro-optically active organic diodes, such as stacked OLEDs, a desire for alternative connection layers, in particular such of good transparency and without the need of using metal layers.